The so-called "advancement" of relatively low molecular weight, low-melting or liquid epoxide resins, by reaction with polyfunctional compounds of which the functional groups react with epoxide groups, to give higher molecular weight, higher melting epoxide resins, is known. Such an advancement is intended to improve or modify, in the desired direction, the technical processing properties for certain end uses. For some end uses, for example in sintering powders and compression molding powders, an increase in the softening point or melting point can be desirable. The advancement produces, in parallel to the increase in size of the molecule, a lowering of the epoxide group content per kilogram of resin and hence a reduction in the reactivity. This has an advantageous effect, for example, when using the product as a casting and impregnating resin, in that the shrinkage or reaction becomes less and reduces the danger of cavity formation, especially in the case of larger castings.
Epoxide resins of relatively high molecular weight, and correspondingly lower epoxide content, can be manufactured in a single stage, by using a smaller storchiometric excess of epichlorohydrin than is used in the manufacture of liquid polyglycidyl ethers. An example of this is the condensation of epichlorohydrin with a polyhydric phenol, such as diomethane [2,2-bis(p-hydrophenyl)propane], in the presence of alkali. This process, however, suffers from the disadvantage that the sodium chloride produced during the condensation is difficult to wash out of the solid epoxide resins thus obtained. Furthermore, the products are as, a rule, very inhomogeneous in their composition, and contain major proportions of branched or partially crosslinked products. The disadvantages described above can be avoided, to a large extent, by manufacturing, in a first stage, low molecular weight liquid polyglycidyl ethers, which are of relatively homogeneous composition and from which sodium chloride and excess alkali can easily be washed out, and subjecting the products thus obtained to a controlled advancement reaction in a second stage. Such processes are, for example, described in U.S. Pat. Nos. 2,615,008 and 3,006,892. In these, dihydric phenols such as diomethane, dicarboxylic acids, or their anhydrides, are primarily used for the advancement.
When using dicarboxylic acids or dicarboxylic acid anhydrides, the storage stability of the advanced epoxide resins is frequently inadequate, because these compounds are active cross-linking agents or curing agents for the epoxide resins, and crosslinking reactions with free hydroxyl groups of the epoxide resin are possible even when less than stoichiometric amounts are used. Diphenols, in the advancement which has been preferred in industry, do not decrease storage stability. However, a serious disadvantage of the incorporation of the aromatic ring structure of the diphenol into the molecule of the advanced epoxide resin is an adverse affect on electrical properties, particularly the tracking resistance and arcing resistance. Such resins tend to form carbon-containing tracks during electrical discharges, and are therefore not well suited to high voltage technology.
This disadvantage is particularly serious in the case of the advancement of relatively low molecular weight epoxide resins which themselves do not contain any aromatic rings, for example, glycidyl esters of hydroaromatic dicarboxylic acids such as tetrahydrophthalic and hexahydrophthalic acid, cycloaliphatic polyepoxides of which the epoxide groups are present in the cyclopentane or cyclohexane rings, or heterocyclic nitrogen-containing glycidyl compounds such as N,N'-diglycidyl-5,5-dimethylhydantoin.
These non-aromatic epoxide resins are, as a rule, distinguished by particularly good electrical properties. In contrast to the polyglycidyl ethers of polyphenols, the chain length and the epoxide content of these non-aromatic epoxide resins cannot be varied within wide limits within the framework of a single-stage process. This can only be achieved by a two-stage process or advancement reaction.
If a diphenol is used for the advancement, then the original outstanding electrical properties of the non-aromatic epoxide resins, such as the arcing resistance and tracking resistance, are decisively worsened as a result of the incorporation of aromatic rings into the resin molecule.
U.S. Pat. No. 3,799,894 teaches that instead of diphenols or dicarboxylic acids, certain binuclear N-heterocyclic compounds containing one endocyclic NH group in each nucleus, and, in particular, bis(hydantoin) compounds or bis(dihydrouracil) compounds, can be employed for the advancement. The epoxide resins which have been advanced with the aid of such nitrogen bases show both good storage stability and excellent electrical properties. In the advancement of non-aromatic epoxide resins, the good electrical properties are fully preserved. It also provides a means to improve the electrical properties of relatively low molecular weight polyglycidyl ethers of polyphenols by the advancement with the above-mentioned heterocyclic nitrogen compounds.
This reference further teaches that the non-aromatic epoxide resins employed therein may be trifunctional, such as 1,3.5-tris(.beta.-glycidyloxypropionyl)-hexahydro-s-triazine and triglycidylisocyanurate. Both above-mentioned trifunctional non-aromatic epoxide resins possess three epoxide groups of equal reactivity, and afford partially crosslinked material upon advancement.
U.S. Pat. Nos. 3,821,243, 3,907,719 and 3,975,397 disclose the process for the preparation of binuclear heterocyclic compounds of the formula ##STR3## where n equals 4 or 5, and R.sub.5 is hydrogen or alkyl of 1 to 4 carbon atoms; the triglycidyl compounds derived therefrom; and curable mixture comprising said triglycidyl compound and an epoxy resin curing agent.
U.S. Pat. No. 3,963,667 teaches the advancement of triglycidyl binuclear hydantoin compounds of the formula ##STR4## wherein R.sub.6 and R.sub.7 denote hydrogen or alkyl group of 1 to 4 carbon atoms or R.sub.6 and R.sub.7 together denote a tetramethylene or pentamethylene group and R.sub.8 denotes hydrogen or alkyl group of 1 to 4 atoms with binuclear hydantoins. These adducts are combined with adducts containing amino groups to afford storage-stable molding compositions.
U.S. Pat. No. 4,071,477 discloses hydantoin diglycidyl compounds of the formula ##STR5## wherein R.sub.9 is hydrogen, alkyl containing 1 to 8 carbon atoms or cycloalkyl containing 5 to 6 carbon atoms, and R.sub.10 is alkyl containing 5 to 8 carbon atoms or cycloalkyl containing 5 to 6 carbon atoms.
This reference teaches that these liquid diglycidyl compounds are easily processable as casting and laminating resins, and, when cured, possess excellent resistance to water absorption.